Native "Open With" dialog on Windows (by default).
This allows to use (and associate) given file extensions with any Windows application directly, as users expect.

- moved KRun::KRunPrivate declaration to krun_p.h
- added KRun::KRunPrivate::displayNativeOpenWithDialog() for Windows only in krun_win.cpp
- added a note to KOpenWithDialog that using KRun::displayOpenWithDialog()
is recommended

Also reported in <a href="http://intevation.de/roundup/kolab/issue2766">http://intevation.de/roundup/kolab/issue2766</a>;
(reviewed)